1. Field of the Invention
This invention relates to a polysulfone hollow fiber membrane and the process for making the same; more particularly, to a polysulfone hollow fiber membrane comprising an inner surface of skin layer, a microporous structure inside said membrane, and an outer surface having micropores of an average pore diameter of 500 to 5,000 .ANG. at a fractional surface porosity of 5 to 50%, and the process for making the same.
2. Description of the Prior Art
Recently techniques utilizing membranes having selective permeabilities in the medical field have been markedly developing, and they are being placed one after another in practical use for various filtration media and artificial organs. Particularly, membranes having a form of hollow fiber are advantageous in acquiring large membrane area per volume, and examples of their practical application therefore have been increasing. For materials of the selectively permeable membrane for medical uses, many kinds of polymers have been developed, and there are now in use polymers such as cellulose, cellulose acetate, polyamides, polyacrylonitriles, polymethyl methacrylates, polyvinyl alcohols, polysulfones, polyolefins and the like.
Since polysulfones are excellent in physical and chemical properties such as heat resistance, resistance to acid, resistance to alkali and resistance to oxidation, they have attracted attention in the field of industrial uses and been studied for application to ultrafiltration, reverse osmosis, supporting member for the composite membrane for separating gases, and the like. For example, Unexamined Japanese Patent Laid-Open No. 145379/1979 discloses a "slope-type" polysulfone hollow fiber of both-side skin type having on the inner surface and the outer surface thereof micropores with pore diameters of from 10 to 100 .ANG., which pore diameter gradually increases inward the membrane.
Unexamined Japanese Patent Laid-Open Nos. 105704/1981 and 115602/1981 disclose a polysulfone hollow fiber comprising no microscopically observable pores or open area on both sides of the membrane and an inside structure of bundled-type type, so-called finger-like structure.
Unexamined Japanese Patent Laid-Open No. 114702/1983 discloses a membrane of a sponge-like structure, comprising microslits having an average width of not more than 500 .ANG. on its inner surface and micropores having an average pore diameter of 1,000 to 5,000 .ANG. on its outer surface.
Unexamined Japanese Patent Laid-Open 152704/1981 discloses a polysulfone hollow fiber membrane comprising a skin layer(s) on its inner surface and/or outer surface, and a sponge layer adjacent to the skin layer; Unexamined Japanese Patent Laid-Open No. 82515/1982 discloses a polysulfone hollow fiber membrane comprising a skin layer on its inner surface and cavities adjacent to the skin layer; and Unexamined Japanese Patent Laid-Open No. 154051/1981 discloses an asymmetric polymer membrane comprising a skin and a porous support.
Further known as a hollow fiber membrane for medical use is hemofilter, "DIAFILTER", of Amicon Corporation and "HEMOFLOW" of Fresenius AG.
All of the above-mentioned membranes, however, have drawbacks as described below.
The membrane disclosed in Japanese Patent Laid-Open No. 145379/1979 has a low filtration rate for filtering body fluid on account perhaps of the skin layers being on both sides or of low continuity of pores inside the structure. The membrane disclosed in Japanese Patent Laid-Open Nos. 105704/1981 and 115602/1981 is also unsatisfactory in view of filtration rate for body fluid. The membrane disclosed in Japanese Patent Laid-Open No. 114702/1983 is, though sufficient in filtration rate for body liquid, has too large a permeability for serum albumin, resulting in a flow-away of serum albumin.
The membrane having skin layers on both sides thereof described in 152704/1981 is, though having a low permeability for albumin, which is preferred, low in permeability for inulin and thus inferior in an effect of removing substances having medium molecular weights.
The membrane described in Japanese Patent Laid-Open No. 82515/1981 and the asymmetric membrane described in Japanese Patent Laid-Open No. 154051/1981 have a drawback of low resistance to pressure.
Further, the hemofilter membrane (DIAFILTER) has a low pressure resistance, while it has a dense skin layer on the inner surface thereof, a number of macropores having pore diameters of not less than 10.mu. on the outer surface thereof, and a finger-like structure throughout the inside thereof, thereby rendering the water permeability considerably high.
HEMOFLOW of Fresenius AG is a sponge-like structure membrane having micropores of about 6,000 .ANG. diameter on its outer surface and micropores of about 500 .ANG. diameter on its inner surface; and has a drawback of time-dependent decrease of filtration performance against blood due to the micropores on the inner surface, which causes clogging of proteins in the blood.
There is also known "SULFLUX" of Kanegafuchi Chemical Ind. Co., a sponge-structure membrane having micropores of 1,000 to 3,000 .ANG. diameter on its inner and outer surfaces, which however has permeabilities for proteins such as albumin of at least 90% and is different from the membrane of the present invention.